U.S. Pat. No. 4,509,939 discloses a quarter fold folding device having a quarter fold blade arranged parallel to the direction of forward travel of a signature and intended to introduce the signature between two rotating cylinders parallel to the direction of forward travel, so as to form the longitudinal fold in the signature. The quarter fold blade is suspended from two drive cranks which are rotationally driven parallel in the same direction and at the same speed by means of two additional rotating cranks, each of which is connected to one end of the drive cranks by means of a rotating pivot. In particular, the two additional cranks rotationally driven in the same direction respectively about two first parallel axes, themselves rotationally drive, in the same direction, the drive cranks about the two parallel rotating pivots situated at the outer ends of the two additional cranks. When the additional cranks revolve about the first axes of rotation, the pivots, which are securely fastened, describe, in parallel, circles about these axes. Thus, during their movement, the drive cranks revolve around the pivots and simultaneously describe a circle about the first parallel axes of rotation of the additional cranks. In this manner, the drive cranks carrying the quarter fold blade drive the quarter fold blade in a vertical movement between a bottom position and a top position. Furthermore, U.S. Pat. No. 4,509,939 includes two balancing counterweights rotationally mounted on the rotating pivots, so that when the drive cranks describe the circle about the parallel axes of rotation of the additional cranks by simultaneously revolving about the pivots, the balancing counterweights revolve in the same direction about the pivots and also describe a circle in the same direction about the parallel axes.
A major drawback of such a device is that the balancing counterweights, which are intended to balance the forces generated in a vertical direction by the vertically moving quarter fold blade, themselves generate, in their circular movement, forces in a direction perpendicular to the direction of displacement of the blade, which make the entire device vibrate.